Hunter syndrome, also known as mucopolysaccharidosis Type II (MPS II), is a lysosomal storage disease caused by deficiency or absence of enzyme, iduronate-2-sulfatase (I2S). Iduronate-2-sulfatase is involved in break down and recycle of specific mucopolysaccharides, also known as glycosaminoglycans or GAG. As a result, in Hunter syndrome, GAG builds up in cells throughout the body, which interferes with the normal function of various cells and organs in the body, resulting in a number of serious symptoms. In many cases of Hunter syndrome, there is often a large build-up of GAGs in neurons and meninges of affected individuals, leading to various forms of CNS symptoms, impaired cognitive performance and developmental delays.
Enzyme replacement therapy (ERT) has been used to treat Hunter syndrome. Approved therapy uses intravenous administration of recombinant I2S enzyme. However, intravenously administered enzyme typically does not adequately cross the blood-brain barrier (BBB) into the cells and tissues of the CNS. Therefore, treatment of CNS symptoms of Hunter syndrome has been especially challenging.